Under the 3GPP standards, a NodeB (or an eNB in LTE) is the base station via which mobile devices connect to the core network. Recently the 3GPP standards body has adopted an official architecture and started work on a new standard for home base stations (HNB). Where the home base station is operating in accordance with the (Long Term Evolution) LTE standards, the HNB is sometimes referred to as a HeNB. A similar architecture will also be applied in the WiMAX network. In this case, the home base station is commonly referred to as a femto cell. For simplicity, the present application will use the term HNB to refer to any such home base station and will use the term NodeB generically to refer to other base stations (such as the base station for the macro cell in which a HNB operates). The HNB will provide radio coverage (for example, 3G/4G/WiMAX) within the home, small and medium enterprise, shopping Malls etc and will connect to the core network via a suitable public network (for example via an ADSL link to the Internet) or operator network and in the case of the 3GPP standards, via an optional HNB gateway (HNB-GW) which typically will aggregate traffic from several HNBs.
An HNB may be configured to operate using one of a plurality of access modes, namely: ‘closed’ in which the HNB operates as a closed subscriber group (CSG) cell; ‘hybrid’ in which the HNB operates as a CSG cell where at the same time, non-members are allowed access; and ‘open’ in which the HNB operates as a normal (non-CSG) cell.
The HNB and a mobility management entity (MME) in the core network maintain a list of Closed Subscriber Group Identifiers (CSG-Ids) currently assigned to the given HNB. Each CSG-Id identifies a closed subscriber group comprising a single cell or a collection of cells within a radio network that is open to only a certain group of subscribers. The HNB sends the list of its CSG-Ids to the MME over the S1 interface, the details of which are described in the 3GPP Standards TS 36.413 version 10.2.0 (2011-06-24): “S1 Application Protocol (SLAP) (Release 10)”. The MME stores the received CSG-Id list for the given HNB.
When the mobile device (also known as User Equipment, UE) is a member of a subscriber group, the CSG-Id for that group is added to a white list, which is stored by the UE and the MME. At the time a connection is established between a UE and a HNB, the HNB sends an initial UE message to the MME in the core network. Where the HNB is configured to operate in a closed or hybrid mode, the initial UE message includes a closed subscriber group identifier, CSG-Id, for the UE. The MME performs access control based on the CSG-Id received in the initial UE message and the CSG-Id list received on the S1 interface from the HNB and the CSG-Id list stored for the UE in the Home Subscriber Server (HSS). If the MME can verify that the CSG-Id received in the initial UE message is on the white list for the given UE, and that the given HNB is open to the subscriber group indicated by this CSG-Id, then the UE is allowed access.
Further details of the CSG-Id and the mobility aspects and cell selection procedures are described in the 3GPP Standards TS 23.003 version 10.2.0 (2011-06-15): “Numbering, addressing and identification (Release 10)” and 3GPP Standards TS 22.011 version 8.9.0 (2009-10-01): “Service accessibility (Release 8)” documents.